Chemistry between actors—specifically romantic chemistry between a male and female actor—is a complex thing and for that reason often taken to be undefinable. While it is certainly too complex to put into precise words, this doesn’t mean that nothing profitable can be said about how to achieve “chemistry.” And we can do that by looking at the term we all use to describe it, “Chemistry,” because, as G.K. Chesterton once said

The phrases of the street are not only forcible but subtle: for a figure of speech can often get into a crack too small for a definition.

Chemistry is the study of how chemicals interact with each other, that is, how they react to each other. Some reactions are not that subtle, but most of the ones studied by chemists are. And this is the essence of “chemistry” between actors. It’s all about how they react to each other’s subtleties.

The art of chemistry, which is just faking attraction—the art of acting is, at its core, faking sincerity—consists of doing the things that people who are attracted to each other actually do. This is subtle, and is divisible into three main parts:

1. Being extremely attentive to slight signals from the other
2. Being very attentive to what slight signals one is giving to the other
3. Being around the other person is just positive in its own right

Taking these in turn, the first of them consists of watching the other carefully. That’s not enough in acting, though, since we (the audience) can’t tell what’s going on in the character’s head. Which isn’t even what’s going on in the actor’s head, so even if we were telepathic it wouldn’t work. What the actor needs to do is to signal that he’s paying careful attention. That is done through reactions—mostly subtle—to the signals the other is giving. The reactions can be fake, but the paying attention can’t be. The actor needs to actually watch the other like a hawk and improvise appropriate sorts of minor reactions. A slight sign of interest should result in a slight indication of excitement or happiness. A slight sign of annoyance or frustration should result in a small sign of concern.

Of course, reactions are not necessarily linear. If the man is in a mood to flirt, the woman showing slight frustration might result in the man doubling-down on the frustrating behavior. The point isn’t the particular reaction, but that there is a reaction. (Some of this will be contained in the dialog, which is the job of the screenwriter, not the actors, but a great deal can be done with stance, facial expression, where the actor looks, etc.)

Another important part of this is that the actors do actually have to look at each other. You can’t be attentive to what is the focus of your attention without looking at it. This can be long, lingering looks; it can be sly, furtive looks stolen when there’s the least chance of them being observed. There’s a wide variety in how to do it, but it must actually get done, and it needs to be connected to the actions which follow it.

The second item—being very attentive to what slight signals one is giving the other—will typically manifest itself in a certain amount of awkwardness, though that’s by no means the only possible approach. It’s somewhat inevitable that people who are preoccupied will take very slightly longer to respond to everything. The feeling of extra care being taken in phrasing, at least some of the time is very helpful to communicate this, too. It will get more subtle the older the characters are, of course, since experience simply helps one execute better. Teenagers can stumble over their words; people in their thirties should have only slight delays if we’re to think of them as adults and not old children.

The third item—being around the other person is positive in its own right—needs to manifest in at least a slight uplift in all reactions to everything. If you’ve got a pitbull clamped onto your leg, it’s still better to have a pitbull clamped onto your leg with the love of your life around than when he’s not there. It’s not that people ignore everything—again, you can at best kind of get away with that in teenage puppy love—but that there is some improvement needs to be evident. This is going to be particularly hard to pull off because it means remembering to (slightly) lower the reactions in all scenes without the love interest, but without that the effect won’t be communicated to the audience.

These three things, if done, will go a long way to giving two actors “chemistry”. It’s not easy, but then there is a reason why people are impressed with good actors.